In many prior art nonaqueous dispersion polymerization systems, organic dispersing medium have been utilized having poor solvent properties for the polymer being produced. A dispersing agent was therefore utilized in the organic medium in order to disperse the polymer being formed throughout the medium. These dispersing agents or dispersants were generally polymeric materials such as block copolymers, random copolymers, or homopolymers as described in U.S. Pat. Nos. 4,098,980 and 4,452,960.
Styrene-butadiene rubbers (SBR) have generally been prepared in solvents in which SBR is soluble. However, only SBR's having a styrene content of less than 35% are soluble in hexane or other non-cyclic aliphatic solvents. These higher styrene content SBR polymers are not completely insoluble in the aliphatic solvents, and, in fact, are highly swollen in these solvents. However, SBR's having a styrene content greater than 35% necessarily have been polymerized in aromatic or cycloaliphatic solvents via solution polymerization.
The applicant first determined that certain pre-made dispersing agents can be utilized to conduct the nonaqueous dispersion polymerization production of SBR having a styrene content greater than 35% by weight in aliphatic dispersing medium such as hexane. Although the dispersion process using a pre-made dispersant works well, it has one shortcoming from a practical or commercial scale up point of view. The dispersant must be prepared separately and stored for subsequent use in the polymerization process. Storage tank and transfer lines require a large capital expenditure and the synthesis of the dispersant and transfer time into the polymerization reactor results in higher production costs.
In these first dispersion SBR studies, a single diblock polymer consisting of a short block (5%-10% of total polymer) of hexane soluble polybutadiene and a long block (90%-95% of total) of high styrene content SBR was prepared in the absence of a dispersing agent. Synthesis of this polymer structure in hexane resulted in either extremely viscous cements or the very undesirable phase separation.
It is therefore desirable to provide a dispersion polymerization process in which there is no need to store the dispersing agent prior to the commencement of the dispersion polymerization process.
It is an object of the present invention to provide a polymerization process employing an anionic initiation system to promote the random polymerization of styrene and butadiene monomers in a nonaqueous dispersion into SBR having 35 to 70% by weight of styrene in the presence of a dispersing agent that is prepared in situ, that is, during the polymerization reaction.
Such a nonaqueous dispersion polymerization process offers many advantages including improved stable dispersions, improved heat transfer, energy savings, high polymer concentrations in the reaction medium, increased production capacity, and the production of very high molecular weight polymers; and no need to store the dispersing agent prior to its use.